An insert casing for mounting an electrical device in a floor recess is known from US 2008/054 135 A1. The insert casing comprises a bracket having flexible retaining legs provided with barbs for securing the casing in the floor recess. When a cover is placed thereupon, the retaining legs comprising the barbs are spread outwardly so that the casing is securely fastened in the floor recess by means of the barbs. The drawback of said insert casing is that it has a complex design and is difficult to mount and dismount.